1. Field of the Invention
The present invention relates to an apparatus and method for adjusting an optical system.
2. Description of Related Art
In a conventional optical data recording and reproducing apparatus in which a laser beam is used to record data onto a recording medium, such as a photo magnetic disc (magneto-optical disc), and to read and reproduce the recorded data from the recording medium, it is necessary to converge the laser beam onto the recording medium so as to form a beam spot. The beam spot is made as small as possible to thereby correctly write data onto and read data from the recording medium. However, if the optical axis of the objective lens is not normal to the recording medium, coma occurs. The coma makes it impossible to correctly converge the laser beam onto the recording medium to form a small beam spot, resulting in a restriction of the amplitude of reading signals or an increase of jitters, and thus leading to a failure to correctly reproduce the recorded data.
To this end, it is necessary to precisely adjust the posture of the objective lens so that the optical axis thereof is normal to the recording medium, in the course of the manufacturing process of the optical data recording and reproducing apparatus. The allowable inclination of the optical axis of the objective lens with respect to the recording medium decreases as the numerical aperture (NA) of the objective lens increases. The objective lens used in recent optical data recording and reproducing apparatuses has a large numerical aperture to enhance the data recording density.
The U.S. Pat. No. 5,157,459 has disclosed an inclination adjusting device of an objective lens for adjusting an inclination of the objective lens relative to a recording medium. The adjusting device can be attached to the optical data recording and reproducing apparatus. The device is positioned opposite to the objective lens of the data recording and reproducing apparatus. In other words, adjustment can be performed with a master glass, which corresponds to an optical disc (i.e., the recording medium), positioned opposite to the objective lens. This inclination adjusting device takes in coherent light that is emitted from a laser light source in the optical data recording and reproducing apparatus, and enables the adjustment of the objective lens inclination relative to the master glass in accordance with the interference fringes of the coherent light.
The device, however, cannot adjust the inclination of the objective lens while monitoring the inclination from the recording medium side because a comatic aberration detecting system having a plurality of optical elements is positioned opposite and close to the objective lens. In other words, since there is little space between the comatic aberration detecting system and the objective lens, the comatic aberration detecting system prevents the device from being capable of adjusting the inclination of the objective lens while monitoring the inclination of the objective lens. Due to this inconvenience in prior art, after the inclination of the objective lens is detected in accordance with the interference fringes, the inclination adjusting device has to be removed from the optical data recording and reproducing apparatus to actually adjust the objective lens. This means that it was impossible to adjust the inclination of the objective lens while monitoring the inclination of the objective lens, i.e., whenever an inclination adjustment was carried out, the adjusting device is first attached to the optical data recording and reproducing apparatus, and then removed after the inclination data is obtained to carry out the actual adjustment of the inclination of the objective lens.
The optical system in the optical data recording and reproducing apparatus is usually comprised of various optical elements. It is necessary for the optical system to have an extremely high performance with respect to the diffraction limit (resolution limit). Accordingly, the optical elements which constitute the optical system must be extremely precisely machined or produced. For instance, a single aspherical objective lens, which is provided on opposite sides thereof with aspherical lens surfaces and which has a numerical aperture NA of about 0.55, the wavefront aberration (coma) of the objective lens caused by the deviation of the optical system, etc., would amount to around 0.9 .lambda. (.lambda.=wavelength of light) which is represented by the PV value (difference between the maximum value and the minimum value), assuming that the parallel or transverse deviation (decentering) of one of the aspherical lens surfaces is 10 .mu.m.
To prevent such a large wavefront aberration from occurring, not only must high precision machines be employed to produce or machine the optical elements, but the optical system also must be very carefully assembled. This increases the manufacturing cost of the optical system in the optical data recording and reproducing apparatus. Moreover, the assembling and adjustment of the optical system require a long time and a highly skilled artisan.